Long Term Evolution (LTE)
Third generation (3G) mobile systems, such as, for instance, universal mobile telecommunication systems (UMTS) standardized within the third generation partnership project (3GPP) have been based on wideband code division multiple access (WCDMA) radio access technology. Today, 3G systems are being deployed on a broad scale all around the world. After enhancing this technology by introducing high-speed downlink packet access (HSDPA) and an enhanced uplink, also referred to as high-speed uplink packet access (HSUPA), the next major step in evolution of the UMTS standard has brought the combination of orthogonal frequency division multiplexing (OFDM) for the downlink and single carrier frequency division multiplexing access (SC-FDMA) for the uplink. This system has been named long term evolution (LTE) since it has been intended to cope with future technology evolutions.
The LTE system represents efficient packet based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The downlink will support data modulation schemes QPSK, 16QAM, and 64QAM and the uplink will support QPSK, 16QAM, and at least for some devices also 64QAM, for physical data channel transmissions. The term “downlink” denotes direction from the network to the terminal. The term “uplink” denotes direction from the terminal to the network.
LTE's network access is to be extremely flexible, using a number of defined channel bandwidths between 1.4 and 20 MHz, compared with UMTS terrestrial radio access (UTRA) fixed 5 MHz channels. Spectral efficiency is increased by up to four-fold compared with UTRA, and improvements in architecture and signaling reduce round-trip latency. Multiple Input/Multiple Output (MIMO) antenna technology should enable 10 times as many users per cell as 3GPP's original WCDMA radio access technology. To suit as many frequency band allocation arrangements as possible, both paired (frequency division duplex FDD) and unpaired (time division duplex TDD) band operation is supported. LTE can co-exist with earlier 3GPP radio technologies, even in adjacent channels, and calls can be handed over to and from all 3GPP's previous radio access technologies.
LTE Architecture
The overall architecture of an LTE network is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2.
As can be seen in FIG. 1, the LTE architecture supports interconnection of different radio access networks (RAN) such as UTRAN or GERAN (GSM EDGE Radio Access Network), which are connected to the EPC via the Serving GPRS Support Node (SGSN). In a 3GPP mobile network, the mobile terminal 110 (called User Equipment, UE, or device) is attached to the access network via the Node B (NB) in the UTRAN and via the evolved Node B (eNB) in the E-UTRAN access. The NB and eNB 120 entities are known as base station in other mobile networks. There are two data packet gateways located in the EPS for supporting the UE mobility—Serving Gateway (SGW) 130 and Packet Data Network Gateway 160 (PDN-GW or shortly PGW). Assuming the E-UTRAN access, the eNB entity 120 may be connected through wired lines to one or more SGWs via the S1-U interface (“U” stays for “user plane”) and to the Mobility Management Entity 140 (MME) via the S1-MMME interface. The SGSN 150 and MME 140 are also referred to as serving core network (CN) nodes.
As anticipated above and as depicted in FIG. 2, the E-UTRAN consists of eNodeB 120, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB 120 hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs 120 are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs 120. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME 140 is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE (Release 8)
FIGS. 3 and 4 illustrate the structure of a component carrier in the LTE release 8. The downlink component carrier of a 3GPP LTE Release 8 is subdivided in the time-frequency domain in so-called subframes, each of which is divided into two downlink slots as shown in FIG. 3. A downlink slot corresponding to a time period Tslot is shown in detail in FIGS. 3 and 4 with the reference numeral 320. The first downlink slot of a subframe comprises a control channel region (PDCCH region) within the first OFDM symbol(s). Each subframe consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier.
In particular, the smallest unit of resources that can be assigned by a scheduler is a resource block also called physical resource block (PRB). With reference to FIG. 4, a PRB 330 is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive sub-carriers in the frequency domain. In practice, the downlink resources are assigned in resource block pairs. A resource block pair consists of two resource blocks. It spans NscRB consecutive sub-carriers in the frequency domain and the entire 2·NsymbDL modulation symbols of the subframe in the time domain. NsymbDL may be either 6 or 7 resulting in either 12 or 14 OFDM symbols in total. Consequently, a physical resource block 330 consists of NsymbDL×NscRB resource elements corresponding to one slot in the time domain and 180 kHz in the frequency domain (further details on the downlink resource grid can be found, for example, in 3GPP TS 36.211, “Evolved universal terrestrial radio access (E-UTRA); physical channels and modulations (Release 10)”, version 10.4.0, 2012, Section 6.2, freely available at www.3gpp.org, which is incorporated herein by reference). While it can happen that some resource elements within a resource block or resource block pair are not used even though it has been scheduled, for simplicity of the used terminology still the whole resource block or resource block pair is assigned. Examples for resource elements that are actually not assigned by a scheduler include reference signals, broadcast signals, synchronization signals, and resource elements used for various control signal or channel transmissions.
The number of physical resource blocks NRBDL in downlink depends on the downlink transmission bandwidth configured in the cell and is at present defined in LTE as being from the interval of 6 to 110 (P)RBs. It is common practice in LTE to denote the bandwidth either in units of Hz (e.g. 10 MHz) or in units of resource blocks, e.g. for the downlink case the cell bandwidth can equivalently expressed as e.g. 10 MHz or NRBDL=50RB.
A channel resource may be defined as a “resource block” as exemplary illustrated in FIG. 3 where a multi-carrier communication system, e.g. employing OFDM as for example discussed in the LTE work item of 3GPP, is assumed. More generally, it may be assumed that a resource block designates the smallest resource unit on an air interface of a mobile communication that can be assigned by a scheduler. The dimensions of a resource block may be any combination of time (e.g. time slot, subframe, frame, etc. for time division multiplex (TDM)), frequency (e.g. subband, carrier frequency, etc. for frequency division multiplex (FDM)), code (e.g. spreading code for code division multiplex (CDM)), antenna (e.g. Multiple Input Multiple Output (MIMO)), etc. depending on the access scheme used in the mobile communication system.
The data are mapped onto physical resource blocks by means of pairs of virtual resource blocks. A pair of virtual resource blocks is mapped onto a pair of physical resource blocks. The following two types of virtual resource blocks are defined according to their mapping on the physical resource blocks in LTE downlink: Localised Virtual Resource Block (LVRB) and Distributed Virtual Resource Block (DVRB). In the localised transmission mode using the localised VRBs, the eNB has full control which and how many resource blocks are used, and should use this control usually to pick resource blocks that result in a large spectral efficiency. In most mobile communication systems, this results in adjacent physical resource blocks or multiple clusters of adjacent physical resource blocks for the transmission to a single user equipment, because the radio channel is coherent in the frequency domain, implying that if one physical resource block offers a large spectral efficiency, then it is very likely that an adjacent physical resource block offers a similarly large spectral efficiency. In the distributed transmission mode using the distributed VRBs, the physical resource blocks carrying data for the same UE are distributed across the frequency band in order to hit at least some physical resource blocks that offer a sufficiently large spectral efficiency, thereby obtaining frequency diversity.
In 3GPP LTE Release 8 the downlink control signalling is basically carried by the following three physical channels:
Physical control format indicator channel (PCFICH) for indicating the number of OFDM symbols used for control signalling in a subframe (i.e. the size of the control channel region); Physical hybrid ARQ indicator channel (PHICH) for carrying the downlink ACK/NACK associated with uplink data transmission; and
Physical downlink control channel (PDCCH) for carrying downlink scheduling assignments and uplink scheduling assignments.
The PCFICH is sent from a known position within the control signalling region of a downlink subframe using a known pre-defined modulation and coding scheme. The user equipment decodes the PCFICH in order to obtain information about a size of the control signalling region in a subframe, for instance, the number of OFDM symbols. If the user equipment (UE) is unable to decode the PCFICH or if it obtains an erroneous PCFICH value, it will not be able to correctly decode the L1/L2 control signalling (PDCCH) comprised in the control signalling region, which may result in losing all resource assignments contained therein.
The PDCCH carries control information, such as, for instance, scheduling grants for allocating resources for downlink or uplink data transmission. The PDCCH for the user equipment is transmitted on the first of either one, two or three OFDM symbols according to PCFICH within a subframe.
Physical downlink shared channel (PDSCH) is used to transport user data. PDSCH is mapped to the remaining OFDM symbols within one subframe after PDCCH. The PDSCH resources allocated for one UE are in the units of resource block for each subframe.
Physical uplink shared channel (PUSCH) carries user data. Physical Uplink Control Channel (PUCCH) carries signalling in the uplink direction such as scheduling requests, HARQ positive and negative acknowledgements in response to data packets on PDSCH, and channel state information (CSI).
The term “component carrier” refers to a combination of several resource blocks. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radio-communication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers (component carriers) are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE are in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanism (e.g. barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. A LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Channel Quality Reporting
The principle of link adaptation is fundamental to the design of a radio interface which is efficient for packet-switched data traffic. Unlike the early versions of UMTS (Universal Mobile Telecommunication System), which used fast closed-loop power control to support circuit-switched services with a roughly constant data rate, link adaptation in LTE adjusts the transmitted data rate (modulation scheme and channel coding rate) dynamically to match the prevailing radio channel capacity for each user.
For the downlink data transmissions in LTE, the eNodeB typically selects the modulation scheme and code rate (MCS) depending on a prediction of the downlink channel conditions. An important input to this selection process is the Channel State Information (CSI) feedback transmitted by the User Equipment (UE) in the uplink to the eNodeB.
Channel state information is used in a multi-user communication system, such as for example 3GPP LTE to determine the quality of channel resource(s) for one or more users. In general, in response to the CSI feedback the eNodeB can select between QPSK, 16-QAM and 64-QAM schemes and a wide range of code rates. This CSI information may be used to aid in a multi-user scheduling algorithm to assign channel resources to different users, or to adapt link parameters such as modulation scheme, coding rate or transmit power, so as to exploit the assigned channel resources to its fullest potential.
Accordingly, the resource grants are transmitted from the eNodeB to the UE in downlink control information (DCI) via PDCCH. The downlink control information may be transmitted in different formats, depending on the signaling information necessary. In general, the DCI may include:                a resource block assignment (RBA),        modulation and coding scheme (MCS).        
The DCI may include further information, depending on the signaling information necessary, as also described in Section 9.3.2.3 of the book “LTE: The UMTS Long Term Evolution from theory to practice” by S. Sesia, I. Toufik, M. Baker, Apr. 2009, John Wiley & Sons, ISBN 978-0-470-69716-0, which is incorporated herein by reference. For instance, the DCI may further include HARQ related information such as redundancy version, HARQ process number, or new data indicator; MIMO related information such as pre-coding; power control related information, etc. Other channel quality elements may be the Precoding Matrix Indicator (PMI) and the Rank Indicator (RI). Details about the involved reporting and transmission mechanisms are given in the following specifications to which it is referred for further reading (all documents available at http://www.3gpp.org and incorporated herein by reference):                3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, version 10.0.0, particularly sections 6.3.3, 6.3.4,        3GPP TS 36.212, “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding”, version 10.0.0, particularly sections 5.2.2, 5.2.4, 5.3.3,        3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, version 10.0.1, particularly sections 7.1.7, and 7.2.        
In 3GPP LTE, not all of the above identified channel quality elements are reported at any time. The elements being actually reported depend mainly on the configured reporting mode. It should be noted that 3GPP LTE also supports the transmission of two codewords (i.e. two codewords of user data (transport blocks) may be multiplexed to and transmitted in a single subframe), so that feedback may be given either for one or two codewords. It should be noted that this information is based on 3GPP TS 36.213, section 7.2.1 mentioned above.
The resource block assignment specifies the physical resource blocks which are to be used for the transmission in uplink or downlink.
The modulation and coding scheme defines the modulation scheme employed for the transmission such as QPSK, 16-QAM or 64-QAM. The lower the order of the modulation, the more robust is the transmission. Thus, 64-QAM is typically used when the channel conditions are good. The modulation and coding scheme also defines a code rate for a given modulation. The code rate is chosen depending on the radio link conditions: a lower code rate can be used in poor channel conditions and a higher code rate can be used in the case of good channel conditions. “Good” and “bad” here is used in terms of the signal to noise and interference ratio. The finer adaptation of the code rate is achieved by puncturing or repetition of the generic rate depending on the error correcting coder type.
FIG. 5 shows an example of an MCS table (MCS Table 0) used in LTE release 10 to determine the modulation order (Qm) used in the physical downlink shared channel. The levels between 0 and 9 in downlink usually represent employing of the robust QPSK modulation. In uplink, LTE release 10 foresees an MCS table which essentially has the same structure of the MCS table for the downlink channel. In downlink the QPSK modulation scheme is represented by the MCS levels between 0 and 9 (for more details refer to 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, version 10.0.1, sections 7 and 8, respectively). The remaining levels specify configurations with higher-level modulation schemes. The levels in the MCS table corresponding to the higher indexes (17 to 28) represent the 64QAM modulation scheme, which is not efficiently usable due to modulation order restrictions. The QPSK and 16QAM modulation schemes are also indicated, in the actual scenario, as low-order modulation schemes as compared to the 64QAM modulation scheme. In general with the term low-order modulation scheme has to be understood any modulation order lower than the highest supported modulation order.
The CSI is reported for every component carrier, and, depending on the reporting mode and bandwidth, for different sets of subbands of the component carrier. A channel resource may be defined as a “resource block” as exemplary illustrated in FIG. 4 where a multi-carrier communication system, e.g. employing OFDM as for example discussed in the LTE work item of 3GPP, is assumed. More generally, it may be assumed that a resource block designates the smallest resource unit on an air interface of a mobile communication that can be assigned by a scheduler. The dimensions of a resource block may be any combination of time (e.g. time slot, subframe, frame, etc. for time division multiplex (TDM)), frequency (e.g. subband, carrier frequency, etc. for frequency division multiplex (FDM)), code (e.g. spreading code for code division multiplex (CDM)), antenna (e.g. Multiple Input Multiple Output (MIMO)), etc. depending on the access scheme used in the mobile communication system.
Assuming that the smallest assignable resource unit is a resource block, in the ideal case channel quality information for each and all resource blocks and each and all users should be always available. However, due to constrained capacity of the feedback channel this is most likely not feasible or even impossible. Therefore, reduction or compression techniques are required so as to reduce the channel quality feedback signalling overhead, e.g. by transmitting channel quality information only for a subset of resource blocks for a given user.
In 3GPP LTE, the smallest unit for which channel quality is reported is called a subband, which consists of multiple frequency-adjacent resource blocks.
As described before, user equipments will usually not perform and report CSI measurements on configured but deactivated downlink component carriers but only radio resource management related measurements like RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality). When activating a downlink component carrier, it's important that the eNodeB acquires quickly CSI information for the newly activated component carrier(s) in order to being able to select an appropriate MCS for efficient downlink scheduling. Without CSI information the eNodeB doesn't have knowledge about the user equipment's downlink channel state and would most likely select a too aggressive or too conservative MCS for downlink data transmission, both of which would in turn lead to resource utilization inefficiency due to required retransmissions or unexploited channel capacity.
Heterogeneous Networks
In the coming years, operators will begin deploying a new network architecture termed Heterogeneous Networks (HetNet). A typical HetNet deployment as currently discussed within 3GPP consists of macro and pico cells. Pico cells are formed by low power eNBs that may be advantageously placed at traffic hotspots in order to offload traffic from macro cells. Macro and pico eNBs implement the scheduling independently from each other. The mix of high power macro cells and low power pico cells can provide additional capacity and improved coverage.
Generally a terminal, such as a user equipment, connects to the node with the strongest downlink signal. In FIG. 6A, the area surrounding the low power eNBs and delimited by a solid line edge is the area where the downlink signal of the low power eNB is the strongest. User equipments within this area will connect to the appropriate low power eNB.
In order to expand the uptake area of a low power eNB without increasing its transmission power an offset is added to the received downlink signal strength in the cell-selection mechanism. In this manner the low power eNB can cover a larger uptake area or in other words the Pico Cells are provided with cell rage expansion (CRE). CRE is a means to increase the throughput performance in such deployments. A UE connects to a macro eNB only if the received power is at least G dB larger than the received power from the strongest pico eNB, where G is the semi-statically configured CRE bias. Typical values are expected to range from 0 to 20 dB.
FIG. 6A illustrates such a HetNet scenario where various pico cells are provided in the area of one macro cell. The range expansion zone (CRE) is delimited in FIG. 6 by a dashed edge. The pico cell edge without CRE is delimited by a solid line edge. Various UEs are shown located in the various cells. FIG. 6B schematically illustrates the concept of a HetNet scenario including a macro eNB and a plurality of pico eNB serving respectively a plurality of UEs located in their coverage areas.
A heterogeneous deployment with a range expansion in the range of 3 to 4 dB has been already considered in the LTE release 8. Nevertheless, the applicability of CRE with cell selection offsets of up to 9 dB have currently being considered at RAN1.
However, the additional capacity provided by the smaller cells may be lost due to signal interference experienced by the UEs in the pico cells. The macro eNB is the single dominant interferer for pico UEs, i.e. for UEs being connected to the pico eNB. This is especially true for pico UEs at the cell edge when using CRE.
Furthermore, the interference problem is aggravated when multiple antenna transmissions are used, as will be explained in the following.
Multiple Antenna System
Multiple Input Multiple Output (MIMO) systems form an essential part of LTE in order to achieve the ambitious requirements for throughput and spectral efficiency. Multiple-input and multiple-output is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology. Note that the terms input and output refer to the radio channel carrying the signal, not to the devices having antennas.
From a high-level perspective, MIMO can be sub-divided into three main categories, beamforming, spatial multiplexing and diversity coding.
MIMO transmissions are in general based on precoding which can be seen as multi-stream beamforming, in the narrowest definition. In more general terms, it is considered to be all spatial processing that occurs at the transmitter. Beamforming takes advantage of interference to change the directionality of the transmitted signal. When transmitting, a beamformer controls the phase and relative amplitude of the signal at each transmitter, in order to create a pattern of constructive and destructive interference in the wavefront.
In single-layer beamforming, the same signal is emitted from each of the transmit antennas with appropriate phase (and sometimes gain) weighting such that the signal power is maximized at the receiver input. The benefits of beamforming are to increase the received signal power level, by making signals emitted from different antennas add up constructively, and to reduce the multipath fading effect; this effect is known as beamforming gain. In the absence of scattering, beamforming results in a well defined directional pattern, but in typical cellular deployments conventional beams are not a good analogy. When there are multiple receivers (mobile terminals) in the system, superposition of multiple transmit beams can be performed if the receives have sufficient spatial separation. Precoding for beamforming requires knowledge of channel state information (CSI) at the transmitter in order to provide optimum adaptation to the channel. Note that single-layer beamforming does in general not require multiple receive antennas on the mobile terminal side.
Spatial multiplexing requires multiple transmit and receive antennas. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted on a spatial layer which is mapped onto the set of transmit antennas in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams into (almost) parallel channels. Spatial multiplexing is a very powerful technique for increasing channel capacity at higher signal-to-noise ratios (SNR). The maximum number of spatial streams is limited by the lesser in the number of antennas at the transmitter or receiver. Spatial multiplexing can be used with or without transmit channel knowledge. Spatial multiplexing can also be used for simultaneous transmission to multiple receivers (mobile terminals), known as multi-user MIMO. By scheduling receivers with different spatial signatures, good separability can be assured.
When there is no channel knowledge at the transmitter, diversity coding techniques can be used. In diversity methods, a single data stream (unlike multiple streams in spatial multiplexing) is transmitted, but the signal is coded using techniques called space-time coding. The signal is emitted from each of the transmit antennas with full or near orthogonal coding. Diversity coding exploits the independent fading in the multiple antenna links to enhance signal diversity. Because there is no channel knowledge on the transmitter side, there is no beamforming gain from diversity coding.
Spatial multiplexing can also be combined with beamforming if the channel is known at the transmitter or combined with diversity coding if increased decoding reliability is required.
Intercell Interference and Coordination
Cell-edge users served by a pico eNodeB usually have relatively low received signal strength, especially if they are located at the border of a pico cell with CRE and suffer from strong intercell interference. The major interferer is the eNodeB serving the macro cell in the Heterogeneous Network, which usually transmits subframes at a high transmission power.
In multi-antenna transmissions with precoding on the interferer side, mobile terminals in the interfered cell may be strongly affected by the use of different precoding matrices in the interfering base station.
The basic interference impact factors are:                Very high average interference level        Very high SINR (CQI) estimation uncertainty due to strong interference flashlight effect        
The Interference Flashlight Effect refers to the effect that each precoding matrix that is used by the interfering base station (described by an Interferer Precoding Matrix Indicator—IPMI) yields a different interference power level on the interference victim mobile terminal side. Since the interferer uses different IPMIs at different times (depending on the multiuser scheduling), the interference victim mobile terminal experiences strong interference fluctuations depending on the IPMIs used by the interference source (interferer base station). These fluctuations are known as the flashlight effect and can result in severe uncertainty concerning the interference level estimation on the victim mobile terminal side.
In order to improve the throughput performance of cell-edge mobile terminals, the interference impact has to be reduced on the resource on which these mobile terminals are scheduled for downlink transmission. The objective of Inter-Cell Interference Coordination (ICIC) is to maximize the multi-cell throughput subject to power constraints, inter-cell signaling limitations, fairness objectives and minimum bit rate requirements.
One solution for interference mitigation is to use subframe patterns with different interference statistics. The concept of creating different interference patterns (e.g. different average interference power levels) of different subframe sets is supported by restricted interference measurements on configured subframe sets as specified in 3GPP RAN1:                Reporting processes for different subframe sets (e.g. Almost Blank Subframe (ABS), non-ABS)        Reports are based on average estimated interference level for a reference resource        
The channel quality is reported to the serving base station (eNB) in form of CQI (Channel Quality Indicator) reports which correspond to a quantization of the expected SINR level on the receiver side. However, CQI reports for different subframe sets and provide no information about expected variability of the interference power level (i.e. flashlight effect); only the average interference power level is taken into account.
Importantly, a strong variability of the interference power level (i.e. flashlight effect) can significantly increase the Block Error Rate (BLER) on the receiver side which results in reduced spectral efficiency.
In HetNet scenarios the coverage area of low power nodes (LPNs), such as pico eNodeBs, overlaps with the coverage area of macrocells and this poses the problem of effectively controlling intercell interference. Further, picocells with CRE allow a UE to access the cell with weak receiving power. Low power access together with downlink interference leads to a lower Signal to Interference plus Noise Ratio (SINR). In a network comprising a macrocell and a plurality of picocells, subscribers with access to picocells with CRE are vulnerable to macro-pico interference. In particular, subscribers that accessed a picocell and are located at the border of CRE, suffer from interference caused by the eNodeB of the macrocell transmitting to subscribers that accessed the macrocell.
A solution for reducing intercell interference is the implementation of the concept of almost blank subframes (ABS). This concept has been introduced in 3GPP release 10 as a means for ICIC. FIG. 7 shows the concept of ABS. The idea is that certain subframes are not used for PDSCH transmissions in the macro cells but only contain some necessary signaling signals, such as PSS/SSS, PBCH, CRS, Paging and SIB1 for assuring back compatibility with the previous LTE releases. That results in significantly reduced interference for picocells UEs scheduled in the same subframe, assumed that macro and picocell subframes are aligned in time. The ratio between number of ABS and number of regular subframes is known as ABS ratio. The optimum ABS ratio setting depends on network deployment, UE distribution and traffic load.
A drawback of using ABS for coordinating intercell interference is that these subframes can not be used for data transmission in the macro cell. As a further improvement of the concept of Almost Blank Subframes the use of Low Power ABS (LP-ABS) in the macrocell, to be supported in release 11, is currently discussed at RAN1.
Since the use of ABS in macrocells has the disadvantage that these subframes cannot be used for data transmissions in the macrocell, the concept of ABS has been extended by the concept of subframes with reduced PDSCH transmission power in 3GPP LTE, release 11. In particular, 3GPP RAN1 has agreed that the use of subframes with reduced PDSCH transmission power in macro cells can be used for inter-cell interference coordination (ICIC) in Heterogeneous Network scenarios consisting of macro and picocells with Cell Range Expansion (CRE). These subframes with reduced transmission power are also known as low power almost blank subframes (LP-ABS) and correspond to the ABS of 3GPP LTE, release 10. An example of LP-ABS in 3GPP LTE release 11 is shown in FIG. 8.
As discussed in the previous section, CRE bias values of up to 9 dB are currently considered at RAN1. Simulation studies of typical HetNet deployments revealed that the optimum PDSCH power reduction for LP-ABS in macrocells corresponds approximately to the CRE bias value. Consequently, in HetNet scenarios with picocells having CRE bias values of 9 dB, power reduction values for the LP-ABS of up to 9 dB can be considered. Reductions of 9 dB for the PDSCH transmission power of LP-ABS is an upper value given by the actual restrictions of 3GPP LTE and this value could change in the future.
In contrast to ABS in 3GPP LTE, release 10, PDSCH transmissions are now allowed in macrocells, but only with reduced transmission power and this is done by using LP-ABS.
The inter-cell interference coordination as defined in 3GPP LTE, release 11, is known as Further Enhanced ICIC (FeICIC) where it is currently discussed to introduce semi-static PDSCH power reduction on a given subframe set as a further optimization parameter in addition to the ABS ratio. Details of the downlink power allocation is specified in 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, version 10.0.1, particularly section 5, which is hereby incorporated by reference.
FIG. 9 schematically illustrates a typical subframe structure in 3GPP LTE, release 10 (FIG. 9(a)) and supporting the PDSCH power reduction in release 11 (FIG. 9(b)). A subcarrier in the PDSCH region of the subframe includes common reference symbols (CRS), which are used for mobility measurements and determination of channel state information (CSI) on the UE side. A problem in implementing the above described concepts arises due to the fact that the transmission power of common reference symbols (CRS), cannot be reduced due to backward compatibility restrictions.
Consequently, the PDSCH power reduction results in a power level difference between CRS and resource elements (REs) of the PDSCH (dynamic range in frequency domain) in the LP-ABS. A large power level difference between CRS REs and PDSCH REs results in an increased EVM (Error Vector Magnitude) of the transmitted signal on transmitter (eNB) side where the exact EVM depends on the transmitter implementation. The Error Vector Magnitude is a measure of the difference between the ideal symbols and the measured symbols after equalization. In the context of digitally modulated signals the EVM is a measure of the deviation of the sent signal constellation from the ideal reference because of non-linearity. The non-linearity results in a compression and/or expansion and a rotation of the signal constellation. The EVM is specified in 3GPP TS 36.104, “Evolved Universal Terrestrial Radio Access (E-UTRA): Base Station (BS) Radio Transmission and Reception” version 10.5.0, section 6, which is hereby incorporated by reference.
An increased EVM on the transmitter side basically translates to a reduced SNR (Signal to Noise Ratio) experienced on the receiver side; an estimation of the relation is given by:
            EVM      RMS        ≈                  [                  1          SNR                ]                    1        /        2              =            [                        N          0                          E          S                    ]              1      /      2      (for more details refer to K. M. Gharaibeh, K. G. Gard, M. B. Steer; “Accurate Estimation of Digital Communication System Metrics—SNR, EVM and ρ in a Nonlinear Amplifier Environment”; 64th ARFTG Microwave Measurements Conference, 2004).
The current minimum dynamic range requirements in the frequency domain that every eNB has to support for PDSCH transmissions without exceeding a certain given EVM is specified in the 3GPP TS 36.104, specifically in sections 6.3 to 6.5. In particular the dynamic range is defined as the difference between the transmitted energy per resource element of a PDSCH RE and of a CRS RE. Accordingly, the PDSCH transmissions that can be supported without exceeding a predefined EVM are listed in table 1 below:
TABLE 1ModulationRE power controlscheme used ondynamic range (dB)the RE(down)(up)QPSK (PDCCH)−6+4QPSK (PDSCH)−6+316QAM (PDSCH)−3+364QAM (PDSCH)00
The specified EVM requirements for the different modulation schemes currently supported in 3GPP LTE are specified in table 2 below:
TABLE 2Modulation scheme forRequired EVMPDSCH[%]QPSK17.5%16QAM12.5%64QAM  8%
Hence, the maximum PDSCH power level reductions for the different modulation schemes that can be assumed to be supported by all eNB implementations without exceeding the specified EVM requirement are given by table 3 below:
TABLE 3Modulation scheme forMaximum power reductionPDSCH[dB]QPSK616QAM364QAM0
The result of these restrictions is that high-order modulation schemes (16QAM and 64QAM) cannot be supported efficiently in subframes with low PDSCH transmission power if eNB implementations meet just the specified minimum requirements.
FIG. 10 shows a quantitative analysis of the macro UE throughput in a standard configuration including four picocells per macrocell and 9 dB CRE bias for picocell. This is a typical HetNet scenario as specified in 3GPP LTE TR 36.814: “Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA physical layer aspects”, release 9, which is herein incorporated by reference. Due to restriction requirements for PDSCH transmission in marocells on low power ABS (LP-ABS) the maximum power reductions applicable to the different modulation schemes are those listed in table 3 above. For an ABS ratio of 0.5 the UE experiences a drop in the throughput up to 17% compared to the case without the restrictions due to the utilization of LP-ABS (FIG. 10(a)). The gap further increases if an ABS ratio of 0.7 is considered. In particular, the UE throughput considering a restriction on the modulation schemes experiences a drop up to 25% compared to the UE throughput in the case that no restriction is applied to the usable modulation schemes. These results are shown in FIG. 10(b).
To conclude, the use of low power ABS (LP-ABS) combined with the restrictions specified in the 3GPP LTE, results in a reduction of the throughput at the User Equipment.
To conclude, the use of low power ABS combined with the restrictions specified in the 3GPP LTE, results in an increased EVM at the transmitter side in case of CRS based PDSCH transmissions and in a consequent reduction of the throughput and an increase of the SNR at the User Equipment.
A solution for reducing the EVM is to implement an enhanced macro eNB. In particular, the EVM could be reduced by increasing the dynamic range of the power amplifier in the eNB. The drawback of this solution is that it results in increased implementation cost.
Alternatively, the occurrence of an increased EVM in subframes with reduced PDSCH transmission power can be accepted as a further signal quality degradation in addition to receiver noise, interference and radio channel attenuation and fading. The eNB could schedule PDSCH transmissions with high-order modulation schemes ignoring the high EVM. The effect of that approach is an increased BLER due to the increased EVM.
Still another solution may be the avoiding the use of high-order modulation schemes in subframes with low PDSCH transmission power. Even if an UE reports a high CQI, the eNB would transmit PDSCHs only with low-order modulation schemes. This approach results in a system throughput reduction.